Electrical safety: why the wall socket is not a toy

Everything you've tried so far uses batteries of a few volts. They are gentle: you could touch both ends of an AA battery and feel nothing. The socket in the wall is a different thing altogether, and it deserves real respect.

This lesson is about why, and how electricity at home is made safe.

Why mains electricity is dangerous

Your body is not a perfect insulator. It's mostly salty water, which carries current surprisingly well, especially when wet.

Ohm's law explains the danger. A 1.5 V battery pushes almost no current through your skin. But 230 V or 120 V from a wall socket can push a current through you that is big enough to stop muscles from letting go, upset your heartbeat and cause burns. Currents of just a few hundredths of an amp through the chest can be dangerous.

It is the current that harms, but a high voltage is what pushes a dangerous current through your skin.

The rules

1. Only experiment with batteries. Never with the wall socket.
2. Never poke anything into a socket, and keep sockets covered in homes with young children.
3. Keep water away from anything plugged in. Never touch a switch or plug with wet hands.
4. Don't use damaged cables or plugs. Frayed wires can expose live metal. Tell an adult.
5. Never open a plugged-in device, or one with a big capacitor, such as a TV or microwave. Even when switched off, some store charge.
6. Stay away from power lines and fallen cables. Never fly kites or climb near them.
7. Don't overload sockets. Plugging many heaters into one adapter can overheat the cable.
8. Treat batteries carefully. Don't short-circuit them, crush them, or charge them with the wrong charger. Swollen or hot batteries should be taken to an adult at once.

Fuses and circuit breakers

Overloaded wires get hot, because the heat made is the current squared times the resistance. To stop this, every home circuit has a fuse or circuit breaker that acts as a safety valve.

• A fuse contains a thin wire designed to melt if the current gets too high. This breaks the circuit. Once blown, it has to be replaced.
• A circuit breaker does the same thing with a switch that trips and can be reset.

Remember Watt Wise? When you switched on too many devices at once, the fuse tripped. The fuse isn't being annoying: the fuse is protecting the wires inside the wall from overheating. If a fuse keeps tripping, something is wrong, and the fix is to use less power or call an electrician, never to put in a bigger fuse.

The earth wire and other safeguards

• Many appliances have a third pin, the earth wire. If a fault ever lets a live wire touch the metal case, current has an easy path to the ground instead of through you, and the fuse trips.
• Residual current devices (called RCDs or GFCIs) watch for current that is going somewhere it shouldn't. If they notice even a tiny leak, perhaps through a person, they cut the power in a fraction of a second.
• Insulation (plastic around the wire) and double-insulated devices keep live parts out of reach.

Safe ways to explore

There is lots to discover with safe, low-voltage parts: batteries, small bulbs, LEDs, wire. Our safe circuits activity guide shows how. You'll learn the same science with no risk.